Message-ID: <8cbaef6c7565deed1109fe958291d9e0@walle.cc>
Date:   Thu, 01 Oct 2020 09:38:29 +0200
From:   Michael Walle <michael@...le.cc>
To:     Tudor.Ambarus@...rochip.com
Cc:     linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org,
        vigneshr@...com, richard@....at, boris.brezillon@...labora.com,
        miquel.raynal@...tlin.com
Subject: Re: [PATCH v3] mtd: spi-nor: keep lock bits if they are non-volatile

Hi Tudor,

Am 2020-10-01 09:07, schrieb Tudor.Ambarus@...rochip.com:
>>>> diff --git a/drivers/mtd/spi-nor/core.c b/drivers/mtd/spi-nor/core.c
>>>> index cc68ea84318e..fd1c36d70a13 100644
>>>> --- a/drivers/mtd/spi-nor/core.c
>>>> +++ b/drivers/mtd/spi-nor/core.c
>>>> @@ -2916,20 +2916,38 @@ static int spi_nor_quad_enable(struct 
>>>> spi_nor
>>>> *nor)
>>>>  }
>>>> 
>>>>  /**
>>>> - * spi_nor_unlock_all() - Unlocks the entire flash memory array.
>>>> + * spi_nor_global_unprotect() - Perform a global unprotect of the
>>>> memory area.
>>>>   * @nor:    pointer to a 'struct spi_nor'.
>>>>   *
>>>>   * Some SPI NOR flashes are write protected by default after a
>>>> power-on reset
>>>>   * cycle, in order to avoid inadvertent writes during power-up.
>>>> Backward
>>>>   * compatibility imposes to unlock the entire flash memory array at
>>>> power-up
>>>> - * by default.
>>>> + * by default. Do it only for flashes where the block protection 
>>>> bits
>>>> + * are volatile, this is indicated by SNOR_F_NEED_UNPROTECT.
>>>> + *
>>>> + * We cannot use spi_nor_unlock(nor->params.size) here because 
>>>> there
>>>> are
>>>> + * legacy devices (eg. AT25DF041A) which need a "global unprotect"
>>>> command.
>>>> + * This is done by writing 0b0x0000xx to the status register. This
>>>> will also
>>>> + * work for all other flashes which have these bits mapped to BP0 
>>>> to
>>>> BP3.
>>>> + * The top most bit is ususally some kind of lock bit for the block
>>>> + * protection bits.
>>>>   */
>>>> -static int spi_nor_unlock_all(struct spi_nor *nor)
>>>> +static int spi_nor_global_unprotect(struct spi_nor *nor)
>>>>  {
>>>> -    if (nor->flags & SNOR_F_HAS_LOCK)
>>>> -            return spi_nor_unlock(&nor->mtd, 0, nor->params->size);
>>>> +    int ret;
>>>> 
>>>> -    return 0;
>>>> +    dev_dbg(nor->dev, "unprotecting entire flash\n");
>>>> +    ret = spi_nor_read_sr(nor, nor->bouncebuf);
>>>> +    if (ret)
>>>> +            return ret;
>>>> +
>>>> +    nor->bouncebuf[0] &= ~SR_GLOBAL_UNPROTECT_MASK;
>>>> +
>>>> +    /*
>>>> +     * Don't use spi_nor_write_sr1_and_check() because writing the
>>>> status
>>>> +     * register might fail if the flash is hardware write 
>>>> protected.
>>>> +     */
>>>> +    return spi_nor_write_sr(nor, nor->bouncebuf, 1);
>>>>  }
>>> 
>>> This won't work for all the flashes. You use a GENMASK(5, 2) to clear
>>> the Status Register even for BP0-2 flashes and you end up clearing
>>> BIT(5)
>>> which can lead to side effects.
>>> 
>>> We should instead introduce a 
>>> nor->params->locking_ops->global_unlock()
>>> hook
>>> for the flashes that have special opcodes that unlock all the flash
>>> blocks,
>>> or for the flashes that deviate from the "clear just your BP bits"
>>> rule.
>> 
>> Wouldn't it make more sense to just set params->locking_ops for these
>> flashes
>> to different functions? or even provide a spi_nor_global_unprotect_ops
>> in
>> core.c and these flashes will just set them. there is no individual
>> sector
>> range lock for these chips. just a lock all or nothing.
> 
> I like the idea of having all locking related functions placed in a 
> single
> place, thus the global_unlock() should be inside locking_ops struct.

My point was that this global unlock shouldn't be a special case for the
current spi_nor_unlock() but just another "how to unlock the flash" 
function
and thus should replace the original unlock op. For example, it is also 
likely
that you need a special global lock (i.e. write all 1's).

static int spi_nor_global_unlock()
{
   write_sr(0); /* actually it will be a read-modify write */
}

static int spi_nor_global_lock()
{
   write_sr(0x1c);
}

static int spi_nor_is_global_locked()
{
   return read_sr() & 0x1c;
}

const struct spi_nor_locking_ops spi_nor_sr_locking_ops = {
         .lock = spi_nor_global_unlock,
         .unlock = spi_nor_global_lock,
         .is_locked = spi_nor_is_global_locked,
};

Having the spi_nor_unlock decide what op to choose introduces just
another indirection. Esp. if you think about having support for
individual sector protection which also needs new ops. Btw. to me
it seems that "global (un)lock" is almost always used for the
individual sector protection scheme, i.e. like a shortcut to allow all
sectors be unlocked at once.

> You can update params->locking_ops. If we have vendor specific 
> locking_ops
> we can set them via:
> 	nor->manufacturer->fixups->default_init(nor);
> or if they are flash specific, with a smaller granularity, via:
> 	nor->info->fixups->default_init(nor);

ok.

>> If it is more common and not just one vendor it might also make sense 
>> to
>> add
>> a seperate flag which will then set the locking ops to
>> spi_nor_global_unprotect_ops, also it seems that there have to be two
> 
> I don't like that we have so many flags, so I try to avoid introducing 
> new
> ones as best as I can. Checking for null pointer should suffice.

Yeah, I already guessed that this would be the answer ;)

-michael

Powered by blists - more mailing lists